Although blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art, prior art Ziegler-Natta catalyst systems could only produce ethylene propylene rubber compositions with greater than 30% by weight ethylene at practical, economic polymerization conditions. There exists a need for polymeric materials which have advantageous processing characteristics while still providing suitable end properties to articles formed therefrom, e.g., tensile and impact strength. Copolymers and blends of polymers have been developed to try and meet the above needs. U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers. In U.S. Pat. No. 3,888,949 Chi-Kai Shih, assigned to E I DuPont, shows the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6–20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst which makes copolymers which are non-uniform in compositional distribution and typically broad in molecular weight distribution. Compositional distribution is a property of copolymers where there exists statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later. The presence of intramolecular compositional distribution is described in U.S. Pat. No. 3,888,949 by the use of the term “block” in the description of the polymer where the copolymer is described as having “sequences of different alpha-olefin content.” Within the context of the invention described above the term sequences describes a number of olefin monomer residues catenated together by chemical bonds and obtained by a polymerization procedure.
In U.S. Pat. No. 4,461,872, A. C. L. Su improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system. However, the properties and compositions of the copolymer with respect to either the nature and type of monomers (alpha-olefin containing 6–20 carbon atoms) or the blocky heterogeneous intra/inter molecular distribution of the alpha-olefin in the polymer have not been resolved since the catalysts used for these polymerization of propylene and alpha-olefin are expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
In two successive publications in the journal of Macromolecules, 1989, V 22, pages 3851–3866, J. W. Collette of E. I. DuPont has described blends of isotactic polypropylene and partially atactic polypropylene which have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
In U.S. Pat. Nos. 3,853,969 and 3,378,606, E. G. Kontos discloses the formation of in situ blends of isotactic polypropylene and “stereo block” copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the “stereo block” character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend. In situ blends of isotactic polypropylene and compositionally uniform random ethylene propylene copolymers have poor properties. Moreover, all of these compositions either do not meet all of the desired properties for various applications, and/or involve costly and burdensome process steps to achieve the desired results.
Similar results are anticipated by R. Holzer and K. Mehnert in U.S. Pat. No. 3,262,992 assigned to Hercules wherein the authors disclose that the addition of a stereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. The synthesis of the these copolymers is designed around polymerization conditions where the polymer chains are generated in different compositions of ethylene and propylene achieved by changing, with time, the monomer concentrations in the reactor. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60). The presence of stereoblock character in these polymers is shown by the high melting point of these polymers, which is much greater than the melting point of the second polymer component in the present invention, shown in Table 1, as well as the poor solubility of these hetero block materials, as a function of the ethylene wt % of the material as shown in Table 3.
It would be desirable to produce a blend of a crystalline polymer, hereinafter referred to as the “first polymer component,” and a crystallizable polymer, hereinafter referred to as the “second polymer component”, having advantageous processing characteristics while still providing end products made from the blend composition having the desired properties, i.e., increased tensile strength, elongation, and overall toughness. The first polymer component (abbreviated as “FPC” in the Tables below) and the second polymer component (abbreviated as “SPC” in the Tables below). Indeed, there is a need for an entirely polyolefin composition which is thermally stable, heat resistant, light resistant and generally suitable for thermoplastic elastomer (TPE) applications which has advantageous processing characteristics. Such an entirely polyolefin composition would be most beneficial if the combination of the first polymer component and the second polymer component were significantly different in mechanical properties than the compositionally weighted average of the corresponding properties of first polymer component and second polymer component alone. We anticipate, while not meant to be limited thereby, that the potency of the second polymer component may be increased if it only consists of one or two polyolefin copolymers material defined by uniform intramolecular and intermolecular composition and microstructure.
The term “crystalline,” as used herein for first polymer component, characterizes those polymers which possess high degrees of inter- and intra-molecular order, and which melt higher than 110° C. and preferably higher than 115° C. and have a heat of fusion of at least 75 J/g, as determined by DSC analysis. And, the term “crystallizable,” as used herein for second polymer component, describes those polymers or sequences which are mainly amorphous in the undeformed state, but upon stretching or annealing, crystallization occurs. Crystallization may also occur in the presence of the crystalline polymer such as first polymer component. These polymers have a melting point of less than 105° C. or preferably less than 100° C. and a heat of fusion of less than 75 J/g as determined by DSC.